Concrete has poor physical properties when subject to tensile stress and it is therefore common practice to reinforce it with steel wires and/or rods in order to enhance its performance in load bearing applications where tensile stress is likely to occur. However, the transfer of tensile stress to the reinforcement is never complete and because of the low tensile strength and low extensibility of concrete compared to that of the steel reinforcement, cracking occurs in the concrete, always from that side, face or edge of the member where the tensile stress is developed.
It is this phenomenon of stress cracking which primarily determines the practical load-bearing capability of a given reinforced concrete structural member, because the cracks propagate through the member from that part of it which is subject to the tensile stress, not only weakening the member but more significantly allowing air and moisture to enter and gain access to the reinforcement. Furthermore, the cracks result in increased deflection under load, which in turn increases the tensile strain developed in the concrete and in consequence, the rate of crack development and propagation increases. Long before failure of the member occurs, its load-bearing capability is significantly impaired.
The historical solution to this problem lies in calculated over-design. Faced with a given, safe working load, the design process includes the step of calculating the theoretical ultimate load strength necessary to provide this safe working load. Then the designer must take into account the effects of deflection and stress cracking. Given a maximum allowable deflection under the safe working load, the permissible extent of stress cracking and crack propagation must then be allowed for. In practice this is done by choosing a much bigger section, possibly, with a higher ultimate load strength. A structural member designed on this basis is larger, heavier and uses more concrete than would otherwise be necessary. It may also have the incidental effect of reducing headroom, or increasing the overall height/size of a building or other structure.
Pre-stressing the structural element has its own problems. It is expensive and can be difficult to apply in a controlled manner on a building site. Occasionally it can also be dangerous.